Conformally covariant quantization of Maxwell field in de Sitter space

In this article, we quantize the Maxwell ("massless spin one") de Sitter field in a conformally invariant gauge. This quantization is invariant under the SO$_0(2,4)$ group and consequently under the de Sitter group. We obtain a new de Sitter invariant two-points function which is very simple. Our method relies on the one hand on a geometrical point of view which uses the realization of Minkowski, de Sitter and anti-de Sitter spaces as intersections of the null cone in \setR^6 and a moving plane, and on the other hand on a canonical quantization scheme of the Gupta-Bleuler type.Comment: v2 is is the definitive (improved compare to v1) versio

Single-molecule stochastic resonance

Stochastic resonance (SR) is a well known phenomenon in dynamical systems. It consists of the amplification and optimization of the response of a system assisted by stochastic noise. Here we carry out the first experimental study of SR in single DNA hairpins which exhibit cooperatively folding/unfolding transitions under the action of an applied oscillating mechanical force with optical tweezers. By varying the frequency of the force oscillation, we investigated the folding/unfolding kinetics of DNA hairpins in a periodically driven bistable free-energy potential. We measured several SR quantifiers under varied conditions of the experimental setup such as trap stiffness and length of the molecular handles used for single-molecule manipulation. We find that the signal-to-noise ratio (SNR) of the spectral density of measured fluctuations in molecular extension of the DNA hairpins is a good quantifier of the SR. The frequency dependence of the SNR exhibits a peak at a frequency value given by the resonance matching condition. Finally, we carried out experiments in short hairpins that show how SR might be useful to enhance the detection of conformational molecular transitions of low SNR.Comment: 11 pages, 7 figures, supplementary material (http://prx.aps.org/epaps/PRX/v2/i3/e031012/prx-supp.pdf

Conformally related massless fields in dS, AdS and Minkowski spaces

In this paper we write down the equation for a scalar conformally coupled field simultaneously for de Sitter (dS), anti-de Sitter (AdS) and Minkowski spacetime in d-dimensions. The curvature dependence appears in a very simple way through a conformal factor. As a consequence the process of curvature free limit, including wave functions limit and two-points functions, turns to be a straightforward issue. We determine a set of modes, that we call de Sitter plane waves, which become ordinary plane waves when the curvature vanishes.Comment: 7 pages, 1 figur

Symbiotic performance of Medicago ciliaris lines in association with different Sinorhizobium strains

We analyzed nodulation and nitrogen fixation efficiency for seven lines of Medicago ciliaris in association with two references strains of Sinorhizobium medicae (M104) and S. meliloti (RCR2011). These M. ciliaris lines were collected in Northern-East (Enfidha and Soliman) and Western (Rhayet and Mateur) Tunisian areas. The M. truncatula reference line Jemalong A17 (JA17) was also included. Plants were harvested after a culture period of 60 days. Two quantitative traits were measured at the harvest including the mean number of nodules per plant and the aerial dry weight (ADW). Analysis of variance showed that nodulation and nitrogen fixation efficiency (NFE) were dependent on the effects of line, strain and their interaction. The highest levels were observed for strain (45.07%) and line (53.24%) effects, respectively, for nodulation and NFE. While studied lines showed generally more number of nodules with RCR2011, they were more efficient with M104 strain. No significant difference in number of nodules was detected between M. ciliaris lines and JA17 with M104, whereas RCR2011 was generally most infective with JA17. Overall, JA17 line exhibited the largest NFE with both strains. Furthermore, there was no significant (P>0.05) correlation between nodulation and NFE for M. ciliaris line by strain associations

Is there a trend in extremely high river temperature for the next decades? A case study for France

International audienceAfter 2003's summer heat wave, Electricité de France created a global plan called "heat wave-dryness". In this context, the present study tries to estimate high river temperatures for the next decades, taking into account climatic and anthropogenic evolutions. To do it, a specific methodology based on Extreme Value Theory (EVT) is applied. In particular, a trend analysis of water temperature data is done and included in EVT used. The studied river temperatures consist of mean daily temperatures for 27 years measured near the French power plants (between 1977 and 2003), with four series for the Rhône river, four for the Loire river and a few for other rivers. There are also three series of mean daily temperatures computed by a numerical model. For each series, we have applied statistical extreme value modelling. Because of thermal inertia, the Generalized Extreme Value (GEV) distribution is corrected by the medium cluster length, which represents thermal inertia of water during extremely hot events. The µ and s parameters of the GEV distributions are taken as polynomial or continuous piecewise linear functions of time. The best functions for µ and s parameters are chosen using Akaike criterion based on likelihood and some physical checking. For all series, the trend is positive for µ and not significant for s, over the last 27 years. However, we cannot assign this evolution only to the climatic change for the Rhône river because the river temperature is the resultant of several causes: hydraulic or atmospheric, natural or related to the human activity. For the other rivers, the trend for µ could be assigned to the climatic change more clearly. Furthermore, the sample is too short to provide reliable return levels estimations for return periods exceeding thirty years. Still, quantitative return levels could be compared with physical models for example

DNA sequence homology in Rhizobium meliloti plasmids

International audiencePlasmids were recovered by an alkaline procedure from six symbiotically effective strains of Rhizobium meliloti of diverse geographical origin, reported to harbour only one middle-size large plasmid (ranging from 89 to 143 Megadaltons). Each purified plasmid was digested with eight restriction endonucleases; cleavage patterns were very complex: only KpnI and XbaI gave a limited number of bands. Fingerprints were very different, whatever the restriction enzyme or the geographical origin of the strains. However, Southern DNA-DNA hybridizations revealed that the plasmids showed homologous sequences having a high thermal stability. We gave evidence that some of these sequences are common to all the plasmids of R. meliloti. The biological function of these common sequences is unknown. Hybridization with cloned nitrogen fixation (nif) genes from Klebsiella pneumoniae had demonstrated that nif genes were not located on the middle — size plasmids of R. meliloti studied in this paper

Developing a Macroscopic Mechanistic Model for Low Molecular Weight Diffusion through Polymers in the Rubbery State

Raman microspectroscopy was used to determine the Fickian diffusivity of two families of low molecular weight molecules through amorphous polystyrene in the rubbery state. Different effects of the temperature on diffusivity for each of the families suggested that molecular mobility is controlled by both the volume and flexibility of the diffusing substance when the movement of polymer chains can generate stress induced deformation of molecules. The diffusing molecules were represented as Newtonian spring-bead systems, which allowed us to quantify their flexibility, in function of the vibration frequency of their bonds by reconstructing their theoretical spectra. Results showed that the use of molecular descriptors that take into account flexibility rather than the most stable conformation of the diffusing molecules may improve the description of the diffusion behavior caused by variations in shape and size of the free volumes of the polymeric matrix in the rubbery state

Improving signal-to-noise resolution in single molecule experiments using molecular constructs with short handles

We investigate unfolding/folding force kinetics in DNA hairpins exhibiting two and three states with newly designed short dsDNA handles (29 bp) using optical tweezers. We show how the higher stiffness of the molecular setup moderately enhances the signal-to-noise ratio (SNR) in hopping experiments as compared to conventional long handles constructs (approximately 700 bp). The shorter construct results in a signal of higher SNR and slower folding/unfolding kinetics, thereby facilitating the detection of otherwise fast structural transitions. A novel analysis of the elastic properties of the molecular setup, based on high-bandwidth measurements of force fluctuations along the folded branch, reveals that the highest SNR that can be achieved with short handles is potentially limited by the marked reduction of the effective persistence length and stretch modulus of the short linker complex.Comment: Main paper: 20 pages and 6 figures. Supplementary Material: 25 page

Variability salt stress response analysis of Tunisian natural populations of Medicago truncatula (Fabaceae) using salt response index (SRI) ratio

We evaluated the responses to salt stress of 106 Medicago truncatula lines from 11 Tunisian natural populations collected from areas that varied in soil composition, salinity and water availability. Five references lines were also included in this study. Plants were cultivated in two treatments (0 and 50 mM of NaCl) during a period of 60 days. At harvest, we measured 14 quantitative traits of the aerial and root growth to identify genotypic variability in salt response. Analysis of variance showed that, the response to salt lines within populations was dependent on the effects of treatment, population, line within population and their interaction with maximum value recorded for treatment (93.78%). This study also analyzed heritability of the salt response index (SRI), defined as the ratio between the observed values with and without salt treatment. SRI of most measured traits had high broad-sense heritability (H²). Most of established correlations among SRI values of measured traits were positive. SRI means revealed that, Soliman and Bulla Regia are the most salt-tolerant populations. Based on Ward’s estimated distance, all lines were classified into 4 clusters showing similarity and dissimilarity in response to salt stress. The eco-geographical factors that influence more the variation of SRI of measured traits among populations are assimilated P2O5, % organic matter and carbon and mean annual rainfall. Findings from this study will provide the basis for identifying and breeding of salt-tolerant lines in M. truncatula.Key words: Environmental factors, lines, Medicago truncatula, NaCl stress, populations, quantitative traits

Dynamic force spectroscopy of DNA hairpins. II. Irreversibility and dissipation

We investigate irreversibility and dissipation in single molecules that cooperatively fold/unfold in a two state manner under the action of mechanical force. We apply path thermodynamics to derive analytical expressions for the average dissipated work and the average hopping number in two state systems. It is shown how these quantities only depend on two parameters that characterize the folding/unfolding kinetics of the molecule: the fragility and the coexistence hopping rate. The latter has to be rescaled to take into account the appropriate experimental setup. Finally we carry out pulling experiments with optical tweezers in a specifically designed DNA hairpin that shows two-state cooperative folding. We then use these experimental results to validate our theoretical predictions.Comment: 28 pages, 12 figure